The present invention relates generally to MR imaging and, more particularly, to a method and system of MR data acquisition with fractional readout field of view (FOV) to truncate noisy and artifact producing regions.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Generally, in MR imaging, the phase encode dimension of an FOV is sized to be equal to or smaller than the frequency encode dimension of the FOV. An FOV with a smaller phase encoding dimension is typically referred to as a “fractional phase encoding FOV”. A fractional phase encoding FOV is commonly used when prescribing an MR scan to improve the efficiency of the MR scan. That is, by defining the FOV to be dimensionally smaller in the phase encode direction, fewer phase encoding steps are generally required to image the FOV. However, constraints such as artifact suppression or parallel imaging requirements related to coil geometry can affect the desired phase encode direction and, as such, force an increase in the scan FOV. One skilled in the art will appreciate that an increase in scan FOV, i.e. a larger FOV than desired, decreases spatial resolution and increases the likelihood that the scan volume includes artifact-prone regions of “uninteresting” anatomy. In this regard, the scan efficiency benefits associated with the fractional phase encode FOV are reduced—if not lost.
The imaging of non-relevant or “uninteresting” anatomy refers to the acquisition of data from regions of a subject that are outside the targeted region of interest. For example, bilateral breast scans are frequently done with an axial slab. Since frequency needs to be along the anterior/posterior (A/P) direction to eliminate/reduce artifacts from cardiac motion, axial bilateral breast scans require a relatively large FOV to spatially include both breasts. To cover both breasts, the scan volume invariably includes much of the chest region which is not generally of interest for a bilateral breast scan. In fact, the inclusion of the chest region in the FOV can then degrade spatial resolution in the frequency encode (A/P) direction and add motion artifacts. Furthermore, due to the limited region of breast coil sensitivity, the chest part of the images can be noisy and suffer from artifacts in parallel imaging scans.
A similar drawback can occur in sagittal spine imaging where the desired frequency encoding direction is along the A/P axis of the subject. The desired coverage in the phase encoding direction yields an FOV that includes uninteresting anatomy in the frequency encoding direction. Furthermore, with a coil designed to image the spine, regions far from the coil in the reconstructed image can be relatively noisy and artifact-prone, including artifacts introduced by processing noise with a surface coil intensity correction.
It would therefore be desirable to have a system and method capable of defining an FOV that limits spatial coverage of uninteresting anatomy with reduced noise and artifact susceptibility.